A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
When projecting an image onto a substrate it is desirable to ensure that a substrate held on a substrate table is correctly positioned to receive the projected image. The substrate table is positioned using a positioning system which has six degrees of freedom (X, Y, Z, RX, RY, RZ). For any given position of the substrate table an error in each of the six degrees of freedom will be present. A calibration of the positioning system is performed to measure and record these errors. This calibration allows the substrate table to be accurately positioned during subsequent operation of the lithographic apparatus.
One known method of calibrating the positioning of the substrate table is to image alignment marks onto a substrate in a closely packed arrangement, and then to develop the imaged alignment marks and measure their positions. This method is very time consuming, and may for example require several hours.
Additionally, to increase the accuracy of the calibration process, several sub-calibrations may be performed. For example, separate calibrations may be carried out for different spatial frequency portions of the alignment marks. In such an example, the spatial high frequency portion may be calibrated using so-called “plate maps”, the mid frequency portion may be calibrated using the multi-probe techniques using an alignment sensor on an arrangement of imaged alignment marks, and the low frequency part may be calibrated by measuring a reference substrate as well as using as the above-mentioned imaged alignment marks using an alignment sensor. In some instances, separate calibrations present have to be carried out in separate locations, using different methods and/or apparatuses. Therefore, while using such a calibration process increases the accuracy, it is severely time and resource consuming.